This depends entirely on the concentration of the liquid medication you are trying to draw into the syringe. If you have a question about a medication for your pet, you should… call your vet's office and see if they can help you.

You cannot directly convert milligrams (mg) to cubic centimeters (cc). The milligram is a unit of mass (or weight), whereas the cubic centimeter is a unit of volume..
You can… convert milliliters, however, to cubic centimenters. In fact, they are equivalent; that is, 1 ml = 1 cc.

This is difficult because milligrams (mg) is a unit of mass, and cubic centimeters (cc) is a unit of volume. It can be done if you know what you are measuring. For example,… let's use water. Water has a density of 1gram/cc, so it's easy to calculate with. So, for water 0.8mg = 0.0008cc (= 0.8 cubic millimeters), since 1000mg = 1 gram.

There is no equivalence. A milligram is a measure of mass. A cubic centimetre is a measure of volume. The two measure different things and, according to basic principles of… dimensional analysis, conversion from one to the other is not valid. If you are not convinced, consider a cubic centimetre of air. How many milligrams? Next consider a cubic centimetre of lead. How many milligrams? The masses of equal volumes of the two substances will clearly be very different. So there is no direct conversion between mass and volume: you need to know the density of the substance to enable you to carry out the conversion..